class_docstr: Dummy Decoder that delegates to individual real Decoders.
code: def combine_attns(self, attns):
      result = {}
      for key in attns[0].keys():
          result[key] = torch.stack(
              [attn[key] for attn in attns if attn[key] is not None]
          ).mean(0)
      return result
  
imports: import torch
  import torch.nn
  from onmt.encoders.encoder import EncoderBase
  from onmt.decoders.decoder import DecoderBase
  from onmt.models import NMTModel
  import onmt.model_builder
label_desc: Use image node_id 3 for calling the EnsembleDecoder obj's underlying member method code with example usage: obj.combine_attns(attns) and returns: result